But it would be odd for that to be it as I'm no longer able to do either PPM or CPAN on older PCs in the same network where I had formerly been able to do so.

Speaking as a NetAdmin (and having been on the sharp end also :-)), there is always a possibility that the proxy configuration has changed.
I would go back to the suggestion I made earlier (configure CPAN to use one http, one ftp and one rsync url, start wireshark, try to get a module within CPAN) and see what you get.

The fact that you get a RST says that something in the route is shutting down your connection before completion, normally you should see data transfer and then a FIN, FIN+ACK sequence.

The sending IP address on the RST transmission is your culprit, it will help to know which protocol is being blocked. If you try one at a time in the urllist, you will find it fairly quickly.

Good luck, HTH

Updated: Forgot to put the quote in that I was replying to